How to bake in Cycles to an external file
I am using 2.77 and still don't see that option to bake to an external file,anyway i found this and it's probably what you want  

bpy.context.scene.render.bake.filepath = "/tmp/baked.png"
bpy.context.scene.render.bake.save_mode = 'EXTERNAL'
bpy.context.scene.render.bake.use_split_materials = True # or False

or
bpy.ops.object.bake(filepath="/tmp/baked.png", save_mode='EXTERNAL', use_split_materials=True, ...)

source : blender repo